Feeding frenzy common to active black holes likely petered out long ago

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Scientists used X-ray imagery of the area around Sagittarius A*, the supermassive black hole at the center of our galaxy, to figure out why it's been eating so little. This image of the galactic central region is color-coded to represent different X-ray wavelengths.

WASHINGTON — New X-ray images of the supermassive black hole at the center of the Milky Way have helped astronomers determine why that black hole is starving.

Like most other spiral and elliptical galaxies in the universe, the Milky Way harbors an enormous black hole at its heart, dubbed Sagittarius A*.

Sagittarius A* is about 26,000 light-years away and has a mass 4 million times that of the sun.

New images taken by NASA's Chandra X-ray Observatory, revealed here today at the 215th meeting of the American Astronomical Society, show the X-ray emissions of a section of the galactic center about 120 light-years across.

Not only do these images show "how rich this region really is," but they help solve the mystery of why the black hole doesn't seem to be gobbling up as much material as scientists expect, said Chandra team member Roman Shcherbakov of the Harvard-Smithsonian Center for Astrophysics.

While the black hole itself is not visible, its presence is noted by the gravitational effect it has on surrounding material, and by the bright radiation generated as material is accelerated and superheated before being swallowed.

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In the Chandra images, star clusters in the vicinity of the black hole are visible. Particle-laden "winds" expelled by the stars fuel the black hole. Astronomers have previously calculated that the black hole probably devours only about 1 percent of the winds coming from these stars. One percent sounds like a small amount, but even that small amount of gas — presuming it is being consumed as expected — should make the black hole appear 100 times brighter than it actually does.

The new Chandra images along with revamped models of black hole feeding show that the Milky Way's black hole devours only about 0.01 percent of the stellar winds in its vicinity.

"So the black hole really is starving," Shcherbakov said.

Before these new results, astronomers have been at something of a loss as to explain why the black hole isn't eating as much as they predict it should, though they expect part of the reason it is a fairly quiet black hole is the maturity of the Milky Way. The type of feeding frenzy that takes place with more active galactic black holes likely petered out long ago in the Milky Way.

The discrepancy in the amount of X-ray radiation that astronomers see and what they expect is likely explained by some physics missing from models of black hole feeding, Shcherbakov said.

Astronomers know that as gas spirals in toward the black hole, it heats up. Conduction caused by this heating had previously been left out of models, so Shcherbakov and his colleagues added it in. The conduction causes some of the heat in the gas to travel outwards, reducing the strength of the radiation that results from the black hole's consumption. It also creates pressure that helps some stellar winds avoid the black hole's gravitational grasp altogether.

So sure enough, when conduction was factored in, the X-ray emissions expected from the black hole in the model matched up well with what astronomers actually see. The end result is that the Milky Way's black hole gobbles up 100 times less material than astronomers had previously predicted.